Network Working Group                                            E. Ivov
Internet-Draft                                                 Atlassian
Intended status: Standards Track                             E. Rescorla
Expires: August 6, 2018                                       RTFM, Inc.
                                                               J. Uberti
                                                          P. Saint-Andre
                                                        February 2, 2018

Trickle ICE: Incremental Provisioning of Candidates for the Interactive
               Connectivity Establishment (ICE) Protocol


   This document describes "Trickle ICE", an extension to the
   Interactive Connectivity Establishment (ICE) protocol that enables
   ICE agents to send and receive candidates incrementally rather than
   exchanging complete lists.  With such incremental provisioning, ICE
   agents can begin connectivity checks while they are still gathering
   candidates and considerably shorten the time necessary for ICE
   processing to complete.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 6, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Determining Support for Trickle ICE . . . . . . . . . . . . .   5
   4.  Conveying the Initial ICE Description . . . . . . . . . . . .   6
   5.  Responder Procedures  . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Conveying the Initial Response  . . . . . . . . . . . . .   7
     5.2.  Forming Check Lists and Beginning Connectivity
           Checks  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Initiator Procedures  . . . . . . . . . . . . . . . . . . . .   8
   7.  Performing Connectivity Checks  . . . . . . . . . . . . . . .   8
     7.1.  Scheduling Checks . . . . . . . . . . . . . . . . . . . .   9
     7.2.  Check List and Timer State Updates  . . . . . . . . . . .   9
   8.  Discovering and Conveying Additional Local Candidates . . . .  10
     8.1.  Pairing Newly Learned Candidates and Updating
           Check Lists . . . . . . . . . . . . . . . . . . . . . . .  11
       8.1.1.  Inserting a New Pair in a Check List  . . . . . . . .  12
     8.2.  Announcing End of Candidates  . . . . . . . . . . . . . .  15
   9.  Receiving Additional Remote Candidates  . . . . . . . . . . .  17
   10. Receiving an End-Of-Candidates Indication . . . . . . . . . .  17
   11. Trickle ICE and Peer Reflexive Candidates . . . . . . . . . .  17
   12. Concluding ICE Processing . . . . . . . . . . . . . . . . . .  18
   13. Subsequent Exchanges  . . . . . . . . . . . . . . . . . . . .  18
   14. Unilateral Use of Trickle ICE (Half Trickle)  . . . . . . . .  18
   15. Requirements for Signaling Protocols  . . . . . . . . . . . .  19
   16. Preserving Candidate Order while Trickling  . . . . . . . . .  20
   17. Example Flow  . . . . . . . . . . . . . . . . . . . . . . . .  21
   18. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   19. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   20. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     21.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     21.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Interaction with Regular ICE . . . . . . . . . . . .  24
   Appendix B.  Interaction with ICE Lite  . . . . . . . . . . . . .  25
   Appendix C.  Changes from Earlier Versions  . . . . . . . . . . .  26
     C.1.  Changes from draft-ietf-ice-trickle-14  . . . . . . . . .  26

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     C.2.  Changes from draft-ietf-ice-trickle-13  . . . . . . . . .  26
     C.3.  Changes from draft-ietf-ice-trickle-12  . . . . . . . . .  27
     C.4.  Changes from draft-ietf-ice-trickle-11  . . . . . . . . .  27
     C.5.  Changes from draft-ietf-ice-trickle-10  . . . . . . . . .  27
     C.6.  Changes from draft-ietf-ice-trickle-09  . . . . . . . . .  27
     C.7.  Changes from draft-ietf-ice-trickle-08  . . . . . . . . .  27
     C.8.  Changes from draft-ietf-ice-trickle-07  . . . . . . . . .  27
     C.9.  Changes from draft-ietf-ice-trickle-06  . . . . . . . . .  27
     C.10. Changes from draft-ietf-ice-trickle-05  . . . . . . . . .  28
     C.11. Changes from draft-ietf-ice-trickle-04  . . . . . . . . .  28
     C.12. Changes from draft-ietf-ice-trickle-03  . . . . . . . . .  28
     C.13. Changes from draft-ietf-ice-trickle-02  . . . . . . . . .  28
     C.14. Changes from draft-ietf-ice-trickle-01  . . . . . . . . .  28
     C.15. Changes from draft-ietf-ice-trickle-00  . . . . . . . . .  28
     C.16. Changes from draft-mmusic-trickle-ice-02  . . . . . . . .  29
     C.17. Changes from draft-ivov-01 and draft-mmusic-00  . . . . .  29
     C.18. Changes from draft-ivov-00  . . . . . . . . . . . . . . .  29
     C.19. Changes from draft-rescorla-01  . . . . . . . . . . . . .  30
     C.20. Changes from draft-rescorla-00  . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   The Interactive Connectivity Establishment (ICE) protocol
   [rfc5245bis] describes mechanisms for gathering candidates,
   prioritizing them, choosing default ones, exchanging them with a
   remote party, pairing them, and ordering the candidate pairs into
   check lists.  Once all of these actions have been completed (and only
   then), the parties can begin a phase of connectivity checks and
   eventually select the pair of candidates that will be used in a media
   session or for a given media stream.

   Although the sequence described above has the advantage of being
   relatively straightforward to implement and debug once deployed, it
   can also be rather lengthy.  Candidate gathering often involves
   things like querying STUN [RFC5389] servers and allocating relayed
   candidates at TURN [RFC5766] servers.  All of these actions can be
   delayed for a noticeable amount of time; although they can be run in
   parallel, they still need to respect the pacing requirements from
   [rfc5245bis], which is likely to delay them even further.  Some or
   all of these actions also need be completed by the responder.  Both
   agents would next perform connectivity checks and only then would
   they be ready to begin streaming media.

   These factors can lead to relatively lengthy session establishment
   times and thus to a degraded user experience.

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   This document defines a supplementary mode of operation for ICE
   implementations, known as "Trickle ICE", in which candidates can be
   exchanged incrementally.  This enables ICE agents to exchange
   candidates as soon as an ICE session has been initiated and a
   candidate has become available.  Connectivity checks for a media
   stream can also start as soon as the first candidates for that stream
   become available.

   Trickle ICE can reduce session establishment times in cases where
   connectivity is confirmed for the first exchanged candidates (e.g.,
   where candidates for one of the agents are directly reachable from
   the second agent, such as candidates at a media relay).  Even when
   this is not the case, performing candidate gathering for both agents
   and connectivity checks in parallel can considerably shorten ICE
   processing times.

   It is worth noting that there is quite a bit of operational
   experience with the Trickle ICE technique, going back as far as 2005
   (when the XMPP Jingle extension defined a "dribble mode" as specified
   in [XEP-0176]); this document incorporates feedback from those who
   have implemented and deployed the technique.

   In addition to the basics of Trickle ICE, this document also
   describes how to discover support for Trickle ICE, how regular ICE
   processing needs to be modified when forming and updating check
   lists, and how Trickle ICE implementations interoperate with agents
   that only implement regular ICE processing as defined in

   This specification does not define the usage of Trickle ICE with any
   specific signaling protocol (however, see
   [I-D.ietf-mmusic-trickle-ice-sip] for usage with SIP [RFC3261] and
   [XEP-0176] for usage with XMPP [RFC6120]).  Similarly, it does not
   define Trickle ICE in terms of the Session Description Protocol (SDP)
   [RFC4566] or the offer/answer model [RFC3264] because the technique
   can be and already is used in application protocols that are not tied
   to SDP or to offer/answer semantics.  However, because SDP and the
   offer/answer model are familiar to most readers of this
   specification, some examples in this document use those particulars
   in order to explain the underlying concepts.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

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   This specification makes use of all terminology defined for
   Interactive Connectivity Establishment in [rfc5245bis].  In addition,
   it defines the following terms:

   Full Trickle:  The typical mode of operation for Trickle ICE agents,
      in which the initial ICE description can include any number of
      candidates (even zero candidates) and does not need to include a
      full generation of candidates as in half trickle.

   Generation:  All of the candidates conveyed within an ICE session;
      these are the candidates that are associated with a specific
      local/remote ufrag pair (which will change on ICE restart, if any

   Half Trickle:  A Trickle ICE mode of operation where the initiator
      gathers a full generation of candidates strictly before creating
      and conveying the initial ICE description.  Once conveyed, this
      candidate information can be processed by regular ICE agents,
      which do not require support for this specification.  It also
      allows Trickle ICE capable responders to still gather candidates
      and perform connectivity checks in a non-blocking way, thus
      roughly providing "half" the advantages of Trickle ICE.  The
      mechanism is mostly meant for use in cases where the responder's
      support for Trickle ICE cannot be confirmed prior to conveying the
      initial ICE description.

   ICE Description:  Any session-related (as opposed to candidate-
      related) attributes required to configure an ICE agent.  These
      include but are not limited to the username fragment, password,
      and other attributes.

   Trickled Candidates:  Candidates that a Trickle ICE agent conveys
      after conveying the initial ICE description or responding to the
      initial ICE description, but within the same ICE session.
      Trickled candidates can be conveyed in parallel with candidate
      gathering and connectivity checks.

   Trickling:  The act of conveying trickled candidates.

3.  Determining Support for Trickle ICE

   To fully support Trickle ICE, applications SHOULD incorporate one of
   the following mechanisms to enable implementations to determine
   whether Trickle ICE is supported:

   1.  Provide a capabilities discovery method so that agents can verify
       support of Trickle ICE prior to initiating a session (XMPP's
       Service Discovery [XEP-0030] is one such mechanism).

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   2.  Make support for Trickle ICE mandatory so that user agents can
       assume support.

   If an application protocol does not provide a method of determining
   ahead of time whether Trickle ICE is supported, agents can make use
   of the half trickle procedure described in Section 14.

   Prior to conveying the initial ICE description, agents using
   signaling protocols that support capabilities discovery can attempt
   to verify whether or not the remote party supports Trickle ICE.  If
   an agent determines that the remote party does not support Trickle
   ICE, it MUST fall back to using regular ICE or abandon the entire

   Even if a signaling protocol does not include a capabilities
   discovery method, a user agent can provide an indication within the
   ICE description that it supports Trickle ICE by communicating an ICE
   option of 'trickle'.  This token MUST be provided either at the
   session level or, if at the media stream level, for every media
   stream (an agent MUST NOT specify Trickle ICE support for some media
   streams but not others).  NOTE: The encoding of the 'trickle' ICE
   option, and the message(s) used to carry it to the peer, are protocol
   specific.  The encoding for the Session Description Protocol (SDP)
   [RFC4566] is defined in [I-D.ietf-mmusic-trickle-ice-sip].

   Dedicated discovery semantics and half trickle are needed only prior
   to session initiation.  After a session is established and Trickle
   ICE support is confirmed for both parties, either agent can use full
   trickle for subsequent exchanges.

4.  Conveying the Initial ICE Description

   An initiator can start gathering candidates as soon as it has an
   indication that communication is imminent (e.g., a user interface cue
   or an explicit request to initiate a session).  Unlike in regular
   ICE, in Trickle ICE implementations do not need to gather candidates
   in a blocking manner.  Therefore, unless half trickle is being used,
   the user experience is improved if the initiator generates and
   transmits their initial ICE description as early as possible (thus
   enabling the remote party to start gathering and trickling

   An initiator MAY include any mix of candidates when conveying the
   initial ICE description.  This includes the possibility of conveying
   all the candidates the initiator plans to use (as in half trickle
   mode), conveying only a publicly-reachable IP address (e.g., a
   candidate at a media relay that is known to not be behind a
   firewall), or conveying no candidates at all (in which case the

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   initiator can obtain the responder's initial candidate list sooner
   and the responder can begin candidate gathering more quickly).

   Methods for calculating priorities and foundations, as well as
   determining redundancy of candidates, work just as with regular ICE
   (with the exception of pruning of duplicate peer reflexive candidates
   as described under Section 5.2).

5.  Responder Procedures

   When a responder receives the initial ICE description, it will first
   check if the ICE description or initiator indicates support for
   Trickle ICE as explained in Section 3.  If this is not the case, the
   responder MUST process the initial ICE description according to
   regular ICE procedures [rfc5245bis] (or, if no ICE support is
   detected at all, according to relevant processing rules for the
   underlying signaling protocol, such as offer/answer processing rules

   If support for Trickle ICE is confirmed, a responder will
   automatically assume support for regular ICE as well.  Specifically,
   the rules from [rfc5245bis] would imply that ICE itself is not
   supported if the initial ICE description includes no candidates;
   however, such a conclusion is not warranted if the responder can
   confirm that the initiator supports Trickle ICE; in this case,
   fallback to non-ICE processing rules is not necessary.

   If the initial ICE description indicates support for Trickle ICE, the
   responder will determine its role and start gathering and
   prioritizing candidates; while doing so, it will also respond by
   conveying its own ICE description, so that both the initiator and the
   responder can start forming check lists and begin connectivity

5.1.  Conveying the Initial Response

   A responder can respond to the initial ICE description at any point
   while gathering candidates.  Here again the ICE description MAY
   contain any set of candidates, including all candidates or no
   candidates.  (The benefit of including no candidates is to convey the
   ICE description as quickly as possible, so that both parties can
   consider the overall session to be under active negotiation as soon
   as possible.)

   As noted in Section 3, in application protocols that use SDP the
   responder's ICE description can indicate support for Trickle ICE by
   including a token of "trickle" in the ice-options attribute.

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5.2.  Forming Check Lists and Beginning Connectivity Checks

   As soon as the agents have obtained local and remote candidates, both
   agents begin forming candidate pairs, computing candidate pair
   priorities, ordering candidate pairs, pruning duplicate pairs, and
   creating check lists according to regular ICE procedures

   According to those procedures, in order for candidate pairing to be
   possible and for duplicate candidates to be pruned, the candidates
   would need to be provided in the relevant ICE descriptions.  By
   contrast, under Trickle ICE check lists can be empty until candidates
   are conveyed or received.  Therefore Trickle ICE agents handle check
   lists and candidate pairing in a slightly different way than regular
   ICE agents: the agents still form the check lists, but they populate
   the check lists only after they actually have the candidate pairs.
   Every check list is initially placed in the Running state, even if
   there are not yet any candidate pairs in the check list.

   A Trickle ICE agent initially considers all candidate pairs in all
   check lists to be frozen.  It then inspects the first check list and
   attempts to unfreeze all candidate pairs it has received so far that
   belong to the first component on the first media stream (i.e., the
   first media stream that was reported to the ICE implementation from
   the using application).  If that first component of the first media
   stream does not contain candidates for one or more of the currently
   known pair foundations, and if candidate pairs already exist for that
   foundation in one of the following components or media streams, then
   the agent unfreezes the first of those candidate pairs.

   With regard to pruning of duplicate candidate pairs, a Trickle ICE
   agent SHOULD follow a policy of keeping the higher priority candidate
   unless it is peer reflexive.

6.  Initiator Procedures

   When processing the initial ICE description from a responder, the
   initiator follows regular ICE procedures to determine its role, after
   which it forms check lists (as described in Section 5.2) and begins
   connectivity checks.

7.  Performing Connectivity Checks

   For the most part, Trickle ICE agents perform connectivity checks
   following regular ICE procedures.  However, the fact that gathering
   and communicating candidates is asynchronous in Trickle ICE imposes a
   number of changes as described in the following sections.

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7.1.  Scheduling Checks

   The ICE specification [rfc5245bis], Section, specifies that
   an agent will terminate the timer for a triggered check in relation
   to a check list once the agent has exhausted all frozen pairs in the
   check list.  This will not work with Trickle ICE, because more pairs
   will be added to the check list incrementally.

   Therefore, a Trickle ICE agent SHOULD NOT terminate the timer until
   the state of the check list is Completed or Failed as specified
   herein (see Section 8.2).

7.2.  Check List and Timer State Updates

   The ICE specification [rfc5245bis], Section, requires that
   agents update check lists and timer states upon completing a
   connectivity check transaction.  During such an update, regular ICE
   agents would set the state of a check list to Failed if both of the
   following two conditions are satisfied:

   o  all of the pairs in the check list are either in the Failed state
      or Succeeded state; and

   o  there is not a pair in the valid list for each component of the
      media stream.

   With Trickle ICE, the above situation would often occur when
   candidate gathering and trickling are still in progress, even though
   it is quite possible that future checks will succeed.  For this
   reason, Trickle ICE agents add the following conditions to the above

   o  all candidate gathering has completed and the agent is not
      expecting to discover any new local candidates;

   o  the remote agent has conveyed an end-of-candidates indication for
      that check list as described in Section 8.2.

   When a check list is set to Failed as described above, regular ICE
   requires the agent to update all other check lists, placing one pair
   from each check list into the Waiting state and thereby effectively
   placing all remaining check lists into the Running state.  However,
   under Trickle ICE other check lists might still be empty at this
   point (because candidates have not yet been received), and following
   only the rules from regular ICE would prevent the agent from forming
   those check lists (because the state of a check list depends on the
   state of the candidate pairs in that check list, but there might not
   yet by any candidate pairs in a given check list).  In accordance

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   with the ICE specification [rfc5245bis], Section, a Trickle
   ICE agent considers an empty check list to be in the Running state;
   in accordance with Section 8.1.1, when inserting a new candidate pair
   into an empty check list, the agent sets the pair to a state of

8.  Discovering and Conveying Additional Local Candidates

   After candidate information has been conveyed, agents will most
   likely continue discovering new local candidates as STUN, TURN, and
   other non-host candidate gathering mechanisms begin to yield results.
   Whenever an agent discovers such a new candidate it will compute its
   priority, type, foundation and component ID according to regular ICE

   The new candidate is then checked for redundancy against the existing
   list of local candidates.  If its transport address and base match
   those of an existing candidate, it will be considered redundant and
   will be ignored.  This would often happen for server reflexive
   candidates that match the host addresses they were obtained from
   (e.g., when the latter are public IPv4 addresses).  Contrary to
   regular ICE, Trickle ICE agents will consider the new candidate
   redundant regardless of its priority.

   Next the agent "trickles" the newly discovered candidate(s) to the
   remote agent.  The actual delivery of the new candidates is handled
   by a signaling protocol such as SIP or XMPP.  Trickle ICE imposes no
   restrictions on the way this is done (e.g., some applications may
   choose not to trickle updates for server reflexive candidates and
   instead rely on the discovery of peer reflexive ones).

   When candidates are trickled, the signaling protocol MUST deliver
   each candidate to the receiving Trickle ICE implementation not more
   than once and in the same order it was conveyed.  If the signaling
   protocol provides any candidate retransmissions, they need to be
   hidden from the ICE implementation.

   Also, candidate trickling needs to be correlated to a specific ICE
   session, so that if there is an ICE restart, any delayed updates for
   a previous session can be recognized as such and ignored by the
   receiving party.  For example, applications that choose to signal
   candidates via SDP may include a ufrag value in the corresponding
   a=candidate line such as:

     a=candidate:1 1 UDP 2130706431 2001:db8::1 5000 typ host ufrag 8hhY

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   Or as another example, WebRTC implementations may include a ufrag in
   the JavaScript objects that represent candidates.

   Note: The signaling protocol needs to provide a mechanism for both
   parties to indicate and agree on the ICE session in force (as
   identified by the ufrag) so that they have a consistent view of which
   candidates are to be paired.  This is especially important in the
   case of ICE restarts (see Section 13).

   Once the candidate has been conveyed to the remote party, the agent
   checks if any remote candidates are currently known for this same
   stream and component.  If not, the new candidate will simply be added
   to the list of local candidates.

   Otherwise, if the agent has already learned of one or more remote
   candidates for this stream and component, it will begin pairing the
   new local candidates with them and adding the pairs to the existing
   check lists according to their priority.

   Note: A Trickle ICE agent MUST NOT pair a local candidate until it
   has been trickled to the remote agent.

8.1.  Pairing Newly Learned Candidates and Updating Check Lists

   Forming candidate pairs works as described in the ICE specification
   [rfc5245bis].  However, actually adding the new pair to a check list
   happens according to the rules described below.

   If the check list where the pair is to be added already contains the
   maximum number of candidate pairs (100 by default as per
   [rfc5245bis]), the new pair is discarded.

   If the new pair's local candidate is server reflexive, the server
   reflexive candidate MUST be replaced by its base before adding the
   pair to the list.

   Once this is done, the agent examines the check list looking for
   another pair that would be redundant with the new one.  If such a
   pair exists and the type of its remote candidate is not peer
   reflexive, the pair with the higher priority is kept and the one with
   the lower priority is discarded.  If, on the other hand, the type of
   the remote candidate in the pre-existing pair is peer reflexive, the
   agent MUST replace it with the newly formed pair (regardless of their
   respective priorities); this is done by setting the priority of the
   new candidate to the priority of the pre-existing candidate and then
   re-sorting the check list.

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   For all other pairs, including those with a server reflexive local
   candidate that were not found to be redundant, the rules specified in
   the following section apply.

8.1.1.  Inserting a New Pair in a Check List

   Consider the following tabular representation of all check lists in
   an agent (note that initially for one of the foundations, i.e., f5,
   there are no candidate pairs):

   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   | m1 (Audio.RTP)  |  F   |  F   |  F   |      |      |
   | m2 (Audio.RTCP) |  F   |  F   |  F   |  F   |      |
   | m3 (Video.RTP)  |  F   |      |      |      |      |
   | m4 (Video.RTCP) |  F   |      |      |      |      |

                   Figure 1: Example of Check List State

   Each row in the table represents a component for a given media stream
   (e.g., m1 and m2 might be the RTP and RTCP components for audio).
   Each column represents one foundation.  Each cell represents one
   candidate pair.  In the foregoing table, "F" stands for "frozen"; in
   the tables below, "W" stands for "waiting" and "S" stands for

   When an agent commences ICE processing, in accordance with
   Section of [rfc5245bis] it will unfreeze (i.e., place in the
   Waiting state) the topmost candidate pair in every column (i.e., the
   pair with the lowest component ID).  This state is shown in the
   following table, with candidate pairs in the Waiting state marked by

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   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   | m1 (Audio.RTP)  |  W   |  W   |  W   |      |      |
   | m2 (Audio.RTCP) |  F   |  F   |  F   |  W   |      |
   | m3 (Video.RTP)  |  F   |      |      |      |      |
   | m4 (Video.RTCP) |  F   |      |      |      |      |

                    Figure 2: Initial Check List State

   Then, as the checks proceed (see Section of [rfc5245bis]),
   for each pair that enters the Succeeded state (denoted here by "S"),
   the agent will unfreeze all pairs for all media streams with the same
   foundation (e.g., if the pair in column 1, row 1 succeeds then the
   agent will unfreeze the pair in column 1, rows 2, 3, and 4).

   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |      |
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
   | m3 (Video.RTP)  |  W   |      |      |      |  W   |
   | m4 (Video.RTCP) |  W   |      |      |      |  F   |

           Figure 3: Check List State with Unfrozen Media Stream

   Trickle ICE preserves all of these rules as they apply to what we
   might call "static" check list sets.  This implies that if, for some
   reason, a Trickle agent were to begin connectivity checks with all of
   its pairs already present, the way that pair states change is
   indistinguishable from that of a regular ICE agent.

   Of course, the major difference with Trickle ICE is that check list
   sets can be dynamically updated because candidates can arrive after
   connectivity checks have started.  When this happens, an agent sets
   the state of the newly formed pair as described below.

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   Case 1: If the newly formed pair is the topmost pair in its column
   (i.e.  the topmost pair among all the check lists for this
   foundation), set the state to Waiting (e.g., this would be the case
   if the newly formed pair were placed in column 5, row 1).

   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |  W   |
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |      |
   | m3 (Video.RTP)  |  W   |      |      |      |      |
   | m4 (Video.RTCP) |  W   |      |      |      |      |

         Figure 4: Check List State with Newly Formed Pair, Case 1

   Case 2: If the pair immediately above the newly formed pair in its
   column is in the Succeeded state, set the state to Waiting (e.g.,
   this would be the case if the pair in column 5, row 1 succeeded and
   the newly formed pair were placed in column 5, row 2);

   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |  S   |
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |  W   |
   | m3 (Video.RTP)  |  W   |      |      |      |      |
   | m4 (Video.RTCP) |  W   |      |      |      |      |

         Figure 5: Check List State with Newly Formed Pair, Case 2

   Case 3: If there is at least one Succeeded pair in its column above
   the row of the newly formed pair, set the state to Waiting (e.g.,
   this would be the case if the pair in column 5, row 1 succeeded and
   two newly formed pairs were placed in column 5, rows 3 and 4).

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   |                 |  f1  |  f2  |  f3  |  f4  |  f5  |
   | m1 (Audio.RTP)  |  S   |  W   |  W   |      |  S   |
   | m2 (Audio.RTCP) |  W   |  F   |  F   |  W   |  W   |
   | m3 (Video.RTP)  |  W   |      |      |      |  W   |
   | m4 (Video.RTCP) |  W   |      |      |      |  W   |

         Figure 6: Check List State with Newly Formed Pair, Case 3

   Case 4: In all other cases, set the state to Frozen.

8.2.  Announcing End of Candidates

   Once all candidate gathering is completed or expires for an ICE
   session associated with a specific media stream, the agent will
   generate an "end-of-candidates" indication for that session and
   convey it to the remote agent via the signaling channel.  Although
   the exact form of the indication depends on the application protocol,
   the indication MUST specify the generation (ufrag/pwd combination) so
   that an agent can correlate the end-of-candidates indication with a
   particular ICE session.  The indication can be conveyed in the
   following ways:

   o  As part of an initiation request (which would typically be the
      case with the initial ICE description for half trickle)

   o  Along with the last candidate an agent can send for a stream

   o  As a standalone notification (e.g., after STUN Binding requests or
      TURN Allocate requests to a server time out and the agent has is
      not actively gathering candidates)

   Conveying an end-of-candidates indication in a timely manner is
   important in order to avoid ambiguities and speed up the conclusion
   of ICE processing.  In particular:

   o  A controlled Trickle ICE agent SHOULD convey an end-of-candidates
      indication after it has completed gathering for a media stream,
      unless ICE processing terminates before the agent has had a chance
      to complete gathering.

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   o  A controlling agent MAY conclude ICE processing prior to conveying
      end-of-candidates indications for all streams.  However, it is
      RECOMMENDED for a controlling agent to convey end-of-candidates
      indications whenever possible for the sake of consistency and to
      keep middleboxes and controlled agents up-to-date on the state of
      ICE processing.

   When conveying an end-of-candidates indication during trickling
   (rather than as a part of the initial ICE description or a response
   thereto), it is the responsibility of the using protocol to define
   methods for relating the indication to one or more specific media

   Receiving an end-of-candidates indication enables an agent to update
   check list states and, in case valid pairs do not exist for every
   component in every media stream, determine that ICE processing has
   failed.  It also enables an agent to speed up the conclusion of ICE
   processing when a candidate pair has been validated but it involves
   the use of lower-preference transports such as TURN.  In such
   situations, an implementation MAY choose to wait and see if higher-
   priority candidates are received; in this case the end-of-candidates
   indication provides a notification that such candidates are not

   An agent MAY also choose to generate an end-of-candidates indication
   before candidate gathering has actually completed, if the agent
   determines that gathering has continued for more than an acceptable
   period of time.  However, an agent MUST NOT convey any more
   candidates after it has conveyed an end-of-candidates indication.

   When performing half trickle, an agent SHOULD convey an end-of-
   candidates indication together with its initial ICE description
   unless it is planning to potentially trickle additional candidates
   (e.g., in case the remote party turns out to support Trickle ICE).

   After an agent conveys the end-of-candidates indication, it will
   update the state of the corresponding check list as explained in
   Section 7.2.  Past that point, an agent MUST NOT trickle any new
   candidates within this ICE session.  After an agent has received an
   end-of-candidates indication, it MUST also ignore any newly received
   candidates for that media stream or media session.  Therefore, adding
   new candidates to the negotiation is possible only through an ICE
   restart (see Section 13).

   This specification does not override regular ICE semantics for
   concluding ICE processing.  Therefore, even if end-of-candidates
   indications are conveyed, an agent will still need to go through pair
   nomination.  Also, if pairs have been nominated for components and

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   media streams, ICE processing MAY still conclude even if end-of-
   candidates indications have not been received for all streams.

9.  Receiving Additional Remote Candidates

   At any time during ICE processing, a Trickle ICE agent might receive
   new candidates from the remote agent.  When this happens and no local
   candidates are currently known for this same stream, the new remote
   candidates are added to the list of remote candidates.

   Otherwise, the new candidates are used for forming candidate pairs
   with the pool of local candidates and they are added to the local
   check lists as described in Section 8.1.

   Once the remote agent has completed candidate gathering, it will
   convey an end-of-candidates indication.  Upon receiving such an
   indication, the local agent MUST update check list states as per
   Section 7.2.  This might lead to some check lists being marked as

10.  Receiving an End-Of-Candidates Indication

   When an agent receives an end-of-candidates indication for a specific
   media stream, it will update the state of the relevant check list as
   per Section 7.2.  If the check list is still in the Active state
   after the update, the agent will persist the fact that an end-of-
   candidates indication has been received and take it into account in
   future updates to the check list.

11.  Trickle ICE and Peer Reflexive Candidates

   Even though Trickle ICE does not explicitly modify the procedures for
   handling peer-reflexive candidates, use of Trickle ICE can have an
   impact on how they are processed.  With Trickle ICE, it is possible
   that server reflexive candidates can be discovered as peer reflexive
   in cases where incoming connectivity checks are received from these
   candidates before the trickle updates that carry them.

   While this would certainly increase the number of cases where ICE
   processing nominates and selects candidates discovered as peer-
   reflexive, it does not require any change in processing.

   It is also likely that some applications would prefer not to trickle
   server reflexive candidates to entities that are known to be publicly
   accessible and where sending a direct STUN binding request is likely
   to reach the destination faster than the trickle update that travels
   through the signaling path.

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12.  Concluding ICE Processing

   This specification does not directly modify the procedures for ending
   ICE processing described in Section 8 of [rfc5245bis], and Trickle
   ICE implementations follow the same rules.

13.  Subsequent Exchanges

   Before conveying an end-of-candidates indication, either agent MAY
   convey subsequent candidate information at any time allowed by the
   signaling protocol in use.  When this happens, agents will use
   [rfc5245bis] semantics to determine whether or not the new candidate
   information require an ICE restart.  If an ICE restart occurs, the
   agents can assume that Trickle ICE is still supported if support was
   determined previously, and thus can engage in Trickle ICE behavior as
   they would in an initial exchange of ICE descriptions where support
   was determined through a capabilities discovery method.

14.  Unilateral Use of Trickle ICE (Half Trickle)

   In half trickle mode, the initiator conveys the initial ICE
   description with a full generation of candidates.  This ensures that
   the ICE description can be processed by a regular ICE responder and
   is mostly meant for use in cases where support for Trickle ICE cannot
   be confirmed prior to conveying the initial ICE description.  The
   initial ICE description indicate support for Trickle ICE, which means
   the responder can respond with something less than a full generation
   of candidates and then trickle the rest.  The initial ICE description
   for half trickle would typically contain an end-of-candidates
   indication, although this is not mandatory because if trickle support
   is confirmed then the initiator can choose to trickle additional
   candidates before it conveys an end-of-candidates indication.

   The half trickle mechanism can be used in cases where there is no way
   for an agent to verify in advance whether a remote party supports
   Trickle ICE.  Because the initial ICE description contain a full
   generation of candidates, it can thus be handled by a regular ICE
   agent, while still allowing a Trickle ICE agent to use the
   optimization defined in this specification.  This prevents
   negotiation from failing in the former case while still giving
   roughly half the Trickle ICE benefits in the latter (hence the name
   of the mechanism).

   Use of half trickle is only necessary during an initial exchange of
   ICE descriptions.  After both parties have received an ICE
   description from their peer, they can each reliably determine Trickle
   ICE support and use it for all subsequent exchanges.

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   In some instances, using half trickle might bring more than just half
   the improvement in terms of user experience.  This can happen when an
   agent starts gathering candidates upon user interface cues that the
   user will soon be initiating an interaction, such as activity on a
   keypad or the phone going off hook.  This would mean that some or all
   of the candidate gathering could be completed before the agent
   actually needs to convey the candidate information.  Because the
   responder will be able to trickle candidates, both agents will be
   able to start connectivity checks and complete ICE processing earlier
   than with regular ICE and potentially even as early as with full

   However, such anticipation is not always possible.  For example, a
   multipurpose user agent or a WebRTC web page where communication is a
   non-central feature (e.g., calling a support line in case of a
   problem with the main features) would not necessarily have a way of
   distinguishing between call intentions and other user activity.  In
   such cases, using full trickle is most likely to result in an ideal
   user experience.  Even so, using half trickle would be an improvement
   over regular ICE because it would result in a better experience for

15.  Requirements for Signaling Protocols

   In order to fully enable the use of Trickle ICE, this specification
   defines the following requirements for signaling protocols.

   o  A signaling protocol SHOULD provide a way for parties to advertise
      and discover support for Trickle ICE before an ICE session begins
      (see Section 3).

   o  A signaling protocol MUST provide methods for incrementally
      conveying (i.e., "trickling") additional candidates after
      conveying the initial ICE description (see Section 8).

   o  A signaling protocol MUST deliver each trickled candidate not more
      than once and in the same order it was conveyed (see Section 8).

   o  A signaling protocol MUST provide a mechanism for both parties to
      indicate and agree on the ICE session in force (see Section 8).

   o  A signaling protocol MUST provide a way for parties to communicate
      the end-of-candidates indication, which MUST specify the
      particular ICE session to which the indication applies (see
      Section 8.2).

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16.  Preserving Candidate Order while Trickling

   One important aspect of regular ICE is that connectivity checks for a
   specific foundation and component are attempted simultaneously by
   both agents, so that any firewalls or NATs fronting the agents would
   whitelist both endpoints and allow all except for the first
   ("suicide") packets to go through.  This is also important to
   unfreezing candidates at the right time.  While not crucial,
   preserving this behavior in Trickle ICE is likely to improve ICE

   To achieve this, when trickling candidates, agents MUST respect the
   order in which the components and streams appear (implicitly or
   explicitly) as they have been negotiated by means of the relevant
   candidate information.  Therefore candidates for a given component
   MUST NOT be conveyed prior to candidates for a component with a lower
   ID number within the same foundation.  In addition, candidates MUST
   be paired, following the procedures in Section 8.1.1, in the same
   order they are conveyed.

   For example, the following SDP description contains two components
   (RTP and RTCP) and two foundations (host and server reflexive):

     o=jdoe 2890844526 2890842807 IN IP6 2001:db8:a0b:12f0::1
     c=IN IP6 2001:db8:a0b:12f0::1
     t=0 0
     m=audio 5000 RTP/AVP 0
     a=rtpmap:0 PCMU/8000
     a=candidate:1 1 UDP 2130706431 2001:db8:a0b:12f0::1 5000 typ host
     a=candidate:1 2 UDP 2130706431 2001:db8:a0b:12f0::1 5001 typ host
     a=candidate:2 1 UDP 1694498815 2001:db8:a0b:12f0::3 5000 typ srflx
         raddr 2001:db8:a0b:12f0::1 rport 8998
     a=candidate:2 2 UDP 1694498815 2001:db8:a0b:12f0::3 5001 typ srflx
         raddr 2001:db8:a0b:12f0::1 rport 8998

   For this candidate information the RTCP host candidate would not be
   conveyed prior to the RTP host candidate.  Similarly the RTP server
   reflexive candidate would be conveyed together with or prior to the
   RTCP server reflexive candidate.

   Similar considerations apply at the level of media streams in
   addition to foundations; this is covered by the requirement to always

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   start unfreezing candidates starting from the first media stream as
   described under Section 5.2.

17.  Example Flow

   As an example, a typical successful Trickle ICE exchange with a
   signaling protocol that follows the offer/answer model would look
   this way:

           Alice                                            Bob
             |                     Offer                     |
             |            Additional Candidates              |
             |                                               |
             |                     Answer                    |
             |            Additional Candidates              |
             |                                               |
             | Additional Candidates and Connectivity Checks |
             |                                               |
             |<=============== MEDIA FLOWS =================>|

                             Figure 7: Example

18.  IANA Considerations

   IANA is requested to register the following ICE option in the "ICE
   Options" sub-registry of the "Interactive Connectivity Establishment
   (ICE) registry", following the procedures defined in [RFC6336].

   ICE Option:  trickle

   Contact:  IESG,

   Change control:  IESG

   Description:  An ICE option of "trickle" indicates support for
      incremental communication of ICE candidates.

   Reference:  RFC XXXX

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19.  Security Considerations

   This specification inherits most of its semantics from [rfc5245bis]
   and as a result all security considerations described there apply to
   Trickle ICE.

   If the privacy implications of revealing host addresses on an
   endpoint device are a concern, agents can generate ICE descriptions
   that contain no candidates and then only trickle candidates that do
   not reveal host addresses (e.g., relayed candidates).

20.  Acknowledgements

   The authors would like to thank Bernard Aboba, Flemming Andreasen,
   Rajmohan Banavi, Taylor Brandstetter, Philipp Hancke, Christer
   Holmberg, Ari Keranen, Paul Kyzivat, Jonathan Lennox, Enrico Marocco,
   Pal Martinsen, Nils Ohlmeier, Thomas Stach, Peter Thatcher, Martin
   Thomson, Dale R.  Worley, and Brandon Williams for their reviews and
   suggestions on improving this document.  Thanks also to Ari Keranen
   and Peter Thatcher in their role as chairs, and Ben Campbell in his
   role as responsible Area Director.

21.  References

21.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-

              Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
              Connectivity Establishment (ICE): A Protocol for Network
              Address Translator (NAT) Traversal", draft-ietf-ice-
              rfc5245bis-17 (work in progress), February 2018.

21.2.  Informative References

              Ivov, E., Thomas, T., Marocco, E., and C. Holmberg, "A
              Session Initiation Protocol (SIP) usage for Trickle ICE",
              draft-ietf-mmusic-trickle-ice-sip-12 (work in progress),
              December 2017.

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   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002, <https://www.rfc-

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002, <https://www.rfc-

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [RFC4787]  Audet, F., Ed. and C. Jennings, "Network Address
              Translation (NAT) Behavioral Requirements for Unicast
              UDP", BCP 127, RFC 4787, DOI 10.17487/RFC4787, January
              2007, <>.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              DOI 10.17487/RFC5389, October 2008, <https://www.rfc-

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766,
              DOI 10.17487/RFC5766, April 2010, <https://www.rfc-

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, DOI 10.17487/RFC6120,
              March 2011, <>.

   [RFC6336]  Westerlund, M. and C. Perkins, "IANA Registry for
              Interactive Connectivity Establishment (ICE) Options",
              RFC 6336, DOI 10.17487/RFC6336, July 2011,

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              Hildebrand, J., Millard, P., Eatmon, R., and P. Saint-
              Andre, "XEP-0030: Service Discovery", XEP XEP-0030, June

              Beda, J., Ludwig, S., Saint-Andre, P., Hildebrand, J.,
              Egan, S., and R. McQueen, "XEP-0176: Jingle ICE-UDP
              Transport Method", XEP XEP-0176, June 2009.

Appendix A.  Interaction with Regular ICE

   The ICE protocol was designed to be flexible enough to work in and
   adapt to as many network environments as possible.  Despite that
   flexibility, ICE as specified in [rfc5245bis] does not by itself
   support trickle ICE.  This section describes how trickling of
   candidates interacts with ICE.

   [rfc5245bis] describes the conditions required to update check lists
   and timer states while an ICE agent is in the Running state.  These
   conditions are verified upon transaction completion and one of them
   stipulates that:

      If there is not a pair in the valid list for each component of the
      media stream, the state of the check list is set to Failed.

   This could be a problem and cause ICE processing to fail prematurely
   in a number of scenarios.  Consider the following case:

   1.  Alice and Bob are both located in different networks with Network
       Address Translation (NAT).  Alice and Bob themselves have
       different address but both networks use the same private internet
       block (e.g., the "20-bit block" 172.16/12 specified in

   2.  Alice conveys to Bob the candidate which also happens
       to correspond to an existing host on Bob's network.

   3.  Bob creates a check list consisting solely of and
       starts checks.

   4.  These checks reach the host at in Bob's network, which
       responds with an ICMP "port unreachable" error; per [rfc5245bis]
       Bob marks the transaction as Failed.

   At this point the check list only contains Failed candidates and the
   valid list is empty.  This causes the media stream and potentially
   all ICE processing to fail, even though if trickle agents could

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   subsequently convey candidates that would cause previously empty
   check lists to become non-empty.

   A similar race condition would occur if the initial ICE description
   from Alice contain only candidates that can be determined as
   unreachable from any of the candidates that Bob has gathered (e.g.,
   this would be the case if Bob's candidates only contain IPv4
   addresses and the first candidate that he receives from Alice is an
   IPv6 one).

   Another potential problem could arise when a non-trickle ICE
   implementation initiates an interaction with a Trickle ICE
   implementation.  Consider the following case:

   1.  Alice's client has a non-Trickle ICE implementation.

   2.  Bob's client has support for Trickle ICE.

   3.  Alice and Bob are behind NATs with address-dependent filtering

   4.  Bob has two STUN servers but one of them is currently

   After Bob's agent receives Alice's initial ICE description it would
   immediately start connectivity checks.  It would also start gathering
   candidates, which would take a long time because of the unreachable
   STUN server.  By the time Bob's answer is ready and conveyed to
   Alice, Bob's connectivity checks may well have failed: until Alice
   gets Bob's answer, she won't be able to start connectivity checks and
   punch holes in her NAT.  The NAT would hence be filtering Bob's
   checks as originating from an unknown endpoint.

Appendix B.  Interaction with ICE Lite

   The behavior of ICE lite agents that are capable of Trickle ICE does
   not require any particular rules other than those already defined in
   this specification and [rfc5245bis].  This section is hence provided
   only for informational purposes.

   An ICE lite agent would generate candidate information as per
   [rfc5245bis] and would indicate support for Trickle ICE.  Given that
   the candidate information will contain a full generation of
   candidates, it would also be accompanied by an end-of-candidates

   When performing full trickle, a full ICE implementation could
   conveying the initial ICE description or response thereto with no

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   candidates.  After receiving a response that identifies the remote
   agent as an ICE lite implementation, the initiator can choose to not
   trickle any additional candidates.  The same is also true in the case
   when the ICE lite agent initiates the interaction and the full ICE
   agent is the responder.  In these cases the connectivity checks would
   be enough for the ICE lite implementation to discover all potentially
   useful candidates as peer reflexive.  The following example
   illustrates one such ICE session using SDP syntax:

           ICE Lite                                          Bob
              |   Offer (a=ice-lite a=ice-options:trickle)    |
              |                                               |no cand
              |         Answer (a=ice-options:trickle)        |trickling
              |              Connectivity Checks              |
     peer rflx|                                               |
    cand disco|                                               |
              |                                               |
              |<=============== MEDIA FLOWS =================>|

                             Figure 8: Example

   In addition to reducing signaling traffic this approach also removes
   the need to discover STUN bindings or make TURN allocations, which
   may considerably lighten ICE processing.

Appendix C.  Changes from Earlier Versions

   Note to the RFC-Editor: please remove this section prior to
   publication as an RFC.

C.1.  Changes from draft-ietf-ice-trickle-14

   o  Minor modifications to track changes to ICE core.

C.2.  Changes from draft-ietf-ice-trickle-13

   o  Removed independent monitoring of check list "states" of frozen or
      active, since this is handled by placing a check list in the
      Running state defined in ICE core.

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C.3.  Changes from draft-ietf-ice-trickle-12

   o  Specified that the end-of-candidates indication must include the
      generation (ufrag/pwd) to enable association with a particular ICE

   o  Further editorial fixes to address WGLC feedback.

C.4.  Changes from draft-ietf-ice-trickle-11

   o  Editorial and terminological fixes to address WGLC feedback.

C.5.  Changes from draft-ietf-ice-trickle-10

   o  Minor editorial fixes.

C.6.  Changes from draft-ietf-ice-trickle-09

   o  Removed immediate unfreeze upon Fail.

   o  Specified MUST NOT regarding ice-options.

   o  Changed terminology regarding initial ICE parameters to avoid
      implementer confusion.

C.7.  Changes from draft-ietf-ice-trickle-08

   o  Reinstated text about in-order processing of messages as a
      requirement for signaling protocols.

   o  Added IANA registration template for ICE option.

   o  Corrected Case 3 rule in Section 8.1.1 to ensure consistency with
      regular ICE rules.

   o  Added tabular representations to Section 8.1.1 in order to
      illustrate the new pair rules.

C.8.  Changes from draft-ietf-ice-trickle-07

   o  Changed "ICE description" to "candidate information" for
      consistency with 5245bis.

C.9.  Changes from draft-ietf-ice-trickle-06

   o  Addressed editorial feedback from chairs' review.

   o  Clarified terminology regarding generations.

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C.10.  Changes from draft-ietf-ice-trickle-05

   o  Rewrote the text on inserting a new pair into a check list.

C.11.  Changes from draft-ietf-ice-trickle-04

   o  Removed dependency on SDP and offer/answer model.

   o  Removed mentions of aggressive nomination, since it is deprecated
      in 5245bis.

   o  Added section on requirements for signaling protocols.

   o  Clarified terminology.

   o  Addressed various WG feedback.

C.12.  Changes from draft-ietf-ice-trickle-03

   o  Provided more detailed description of unfreezing behavior,
      specifically how to replace pre-existing peer-reflexive candidates
      with higher-priority ones received via trickling.

C.13.  Changes from draft-ietf-ice-trickle-02

   o  Adjusted unfreezing behavior when there are disparate foundations.

C.14.  Changes from draft-ietf-ice-trickle-01

   o  Changed examples to use IPv6.

C.15.  Changes from draft-ietf-ice-trickle-00

   o  Removed dependency on SDP (which is to be provided in a separate

   o  Clarified text about the fact that a check list can be empty if no
      candidates have been sent or received yet.

   o  Clarified wording about check list states so as not to define new
      states for "Active" and "Frozen" because those states are not
      defined for check lists (only for candidate pairs) in ICE core.

   o  Removed open issues list because it was out of date.

   o  Completed a thorough copy edit.

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C.16.  Changes from draft-mmusic-trickle-ice-02

   o  Addressed feedback from Rajmohan Banavi and Brandon Williams.

   o  Clarified text about determining support and about how to proceed
      if it can be determined that the answering agent does not support
      Trickle ICE.

   o  Clarified text about check list and timer updates.

   o  Clarified when it is appropriate to use half trickle or to send no
      candidates in an offer or answer.

   o  Updated the list of open issues.

C.17.  Changes from draft-ivov-01 and draft-mmusic-00

   o  Added a requirement to trickle candidates by order of components
      to avoid deadlocks in the unfreezing algorithm.

   o  Added an informative note on peer-reflexive candidates explaining
      that nothing changes for them semantically but they do become a
      more likely occurrence for Trickle ICE.

   o  Limit the number of pairs to 100 to comply with 5245.

   o  Added clarifications on the non-importance of how newly discovered
      candidates are trickled/sent to the remote party or if this is
      done at all.

   o  Added transport expectations for trickled candidates as per Dale
      Worley's recommendation.

C.18.  Changes from draft-ivov-00

   o  Specified that end-of-candidates is a media level attribute which
      can of course appear as session level, which is equivalent to
      having it appear in all m-lines.  Also made end-of-candidates
      optional for cases such as aggressive nomination for controlled

   o  Added an example for ICE lite and Trickle ICE to illustrate how,
      when talking to an ICE lite agent doesn't need to send or even
      discover any candidates.

   o  Added an example for ICE lite and Trickle ICE to illustrate how,
      when talking to an ICE lite agent doesn't need to send or even
      discover any candidates.

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   o  Added wording that explicitly states ICE lite agents have to be
      prepared to receive no candidates over signaling and that they
      should not freak out if this happens.  (Closed the corresponding
      open issue).

   o  It is now mandatory to use MID when trickling candidates and using
      m-line indexes is no longer allowed.

   o  Replaced use of to IP6 :: in order to avoid potential
      issues with RFC2543 SDP libraries that interpret as an on-
      hold operation.  Also changed the port number here from 1 to 9
      since it already has a more appropriate meaning.  (Port change
      suggested by Jonathan Lennox).

   o  Closed the Open Issue about use about what to do with cands
      received after end-of-cands.  Solution: ignore, do an ICE restart
      if you want to add something.

   o  Added more terminology, including trickling, trickled candidates,
      half trickle, full trickle,

   o  Added a reference to the SIP usage for Trickle ICE as requested at
      the Boston interim.

C.19.  Changes from draft-rescorla-01

   o  Brought back explicit use of Offer/Answer.  There are no more
      attempts to try to do this in an O/A independent way.  Also
      removed the use of ICE Descriptions.

   o  Added SDP specification for trickled candidates, the trickle
      option and addresses in m-lines, and end-of-candidates.

   o  Support and Discovery.  Changed that section to be less abstract.
      As discussed in IETF85, the draft now says implementations and
      usages need to either determine support in advance and directly
      use trickle, or do half trickle.  Removed suggestion about use of
      discovery in SIP or about letting implementing protocols do what
      they want.

   o  Defined Half Trickle.  Added a section that says how it works.
      Mentioned that it only needs to happen in the first o/a (not
      necessary in updates), and added Jonathan's comment about how it
      could, in some cases, offer more than half the improvement if you
      can pre-gather part or all of your candidates before the user
      actually presses the call button.

   o  Added a short section about subsequent offer/answer exchanges.

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   o  Added a short section about interactions with ICE Lite

   o  Added two new entries to the open issues section.

C.20.  Changes from draft-rescorla-00

   o  Relaxed requirements about verifying support following a
      discussion on MMUSIC.

   o  Introduced ICE descriptions in order to remove ambiguous use of
      3264 language and inappropriate references to offers and answers.

   o  Removed inappropriate assumption of adoption by RTCWEB pointed out
      by Martin Thomson.

Authors' Addresses

   Emil Ivov
   303 Colorado Street, #1600
   Austin, TX  78701

   Phone: +1-512-640-3000

   Eric Rescorla
   RTFM, Inc.
   2064 Edgewood Drive
   Palo Alto, CA  94303

   Phone: +1 650 678 2350

   Justin Uberti
   747 6th St S
   Kirkland, WA  98033

   Phone: +1 857 288 8888

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   Peter Saint-Andre

   Phone: +1 720 256 6756

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